D-Xylonic acid (C5H10O6) is an organic acid that, as set out by Toivari et al. in Appl Microbiol Biotechnol, 2012, 96(1): 1-8, can be used as a precursor for polyamides, polyesters and 1,2,4-butanetriol and thus has high potential for use in the pharmaceutical industry, the food industry and the chemical industry. The following explanations relate to D-xylonate, the salt of D-xylonic acid. D-Xylonate is in the top 30 high-potential precursor chemicals based on 2nd generation renewable raw materials, e.g. pentose-containing hemicelluloses. D-Xylonate is similar to D-gluconate (C6H11O7), which has a global market of 80 kt/year.
D-Xylonate is naturally formed in some bacteria in a two-stage reaction. In the first reaction, D-xylose is oxidized to form D-xylonolactone, specific dehydrogenases being catalytically active here depending on the organism. The D-xylonolactone can then be converted to D-xylonate either by specific lactonases or spontaneously, without an enzyme catalyst. For example, for the species Gluconobacter oxydans and Pseudomonas fragi, high product titers of D-xylonate have been reported (Buchert et al., Applied Microbiology and Biotechnology, 27(4): 333-336, Toivari et al., Appl Microbiol Biotechnol, 2012, 96(1): 1-8).
Furthermore, alternative D-xylonate production strains (e.g. yeast of the species Saccharomyces cerevisiae, bacteria of the species Escherichia coli and fungi of the species Aspergillus niger) are produced by heterologous expression of D-xylose dehydrogenases, e.g. from Caulobacter crescentus, as set out in (Liu et al., BioresourTechnol, 2012, 115: 244-248, Richard et al., 2012, US20120005788 A1 (U.S. Ser. No. 13/256,559), Toivari et al., Metab Eng, 2012, 14(4): 427-436).
In addition to microbial production, D-xylonate can be produced electrochemically (Jokic et al., Journal of Applied Electrochemistry, 21(4): 321-326), enzymatically (Pezzotti et al., Carbohydr Res, 2006, 341(13): 2290-2292) or by chemical oxidation (Isbell et al., Bureau of Standards Journal of Research, 1932, 8(3): 327-338).
It is known that organic acids or the salts thereof can be produced by fermentation of strains of coryneform bacteria, in particular Corynebacterium glutamicum. 
US 2013/0295621 A1 discloses a method for producing D-xylonic acid using genetically modified bacteria.
The production strains known from the prior art that have endogenic D-xylonate synthesis capacity, such as Gluconobacter oxydans, require undefined media for growth, meaning that D-xylonate production becomes significantly more complex, expensive and thus uneconomical.
All the previously described “unnatural” D-xylonate producers are genetically modified organisms within the meaning of the German Genetic Engineering Act (GenTG), Section 3, according to which an organism is genetically modified when it is capable of heterologous expression or it contains genes foreign to this organism, i.e. is recombinantly modified, inter alia various yeasts, fungi and bacteria. This is a drawback for use in certain industrial sectors, such as the food and pharmaceutical industries, due to complex approval procedures.
There is a need to provide a method and a microorganism using which D-xylonate and therefore D-xylonic acid can be produced at high enantiomeric purity and with a high yield. In particular, a microorganism and a method are provided which can be used in industrial applications, in particular in the food industry, the pharmaceutical industry and the chemical industry and readily meet the approval conditions for use in the food industry as well as in the pharmaceutical and chemical industries. The drawbacks associated with genetically modified organisms within the meaning of the German Genetic Engineering Act, Section 3, in particular the drawbacks associated with recombinantly modified organisms, are preferably intended to be avoided. In particular, it is desirable to use simple and cost-effective culture media that are inexpensive and easy to use. The microorganisms used are intended to have high growth rates on defined media, to achieve high biomass yields and to simplify the subsequent product processing.